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When I was in school, I learned (from Democritus) that an atom was similar to a solar system, with the nucleus being the sun, and the electrons being the planets. Of course, there are some differences:

  • The "sun" isn't a single entity, but a collection of protons and nuetrons.

  • Two planets can share an orbit (which might be possible in a solar system too, but it doesn't happen in our solar system).

Is this model still valid? Here are my problems with it:

  • In "Surely You're Joking, Mr Feynman", Richard Feynman implies that electrons are more a theoretical concept than real objects.

  • I have trouble understanding atomic bonds (ionic and covalent) in this model.

  • I also have trouble understanding electron "orbit jumping" in this model, as well as several other things.

Is there a better model for someone learning this for the first time?

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    $\begingroup$ 1st, this is not from Demokrit. Because we do not have original texts, we do not know whether he had ideas to some "structure" of his "atoms". 2nd this model of Bohr was never really "valid", (by todays standards) because it had more flaws than goodies. It was only after DeBroglies ideas, that Bohrs model gained some respect. And to the rest: Ever heard of quantum mechanics? For chemistry use best by Schrödingers equation. $\endgroup$
    – Georg
    Commented Nov 11, 2011 at 14:12
  • $\begingroup$ It's valid in the sense that it's a geometrical picture to help you visualize its scale. It's not valid in the sense that all processes going on at this scale are described by the quantum theory, which is not like gravity at all. $\endgroup$
    – Marton Trencseni
    Commented Nov 11, 2011 at 14:48
  • $\begingroup$ How is that you have problems to understand ionic bond in this model? $\endgroup$
    – arivero
    Commented Nov 11, 2011 at 23:25
  • $\begingroup$ btw, It is definitely not Demokrit. His atoms were sizeless, structure was some internal properties "rithmos, diatigue, trop". Epicurus atoms had some minute size, and Copernicus makes the comparision between the size of an atom and the size of the solar system, but without implying any similarity, nor substructure. $\endgroup$
    – arivero
    Commented Nov 11, 2011 at 23:29

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Yes the model is as valid as it has ever been and no there are not better models for explaining it to someone the first time (IMHO).

As Georg points out, the model wasn't ever mathematically valid; it is simply not possible to translate the relativistic model of a planetary system to atomic structure. If the audience is expected, or intended, to actually use the model to make quantitative predictions, the planetary model is pretty useless...

However, people who are learning about atoms for the first time are almost certainly not going to be expected to use the model to make quantitative predictions. Most school systems in the US introduce the concept of atoms before the age of 14. When you have maybe one hour a day for a few days to talk about atomic structure, it is simply not going to be possible to address the subject with any more detail.

The planetary model neither corresponds with reality nor makes valid predictions, but for a 12 year old kid who has some concept of how things can orbit around eachother the planetary model at least gives some of the right ideas. It allows the student to visualize and differentiate between a nucleus and electrons [nuclear physics]; to conceptualize electron loss, gain, and sharing [chemistry]; the movement of electrons along a material [electricity and magnetism]; and eventually photon emission and absorption [optics].

Considering that only a very small number of these students will continue on to learn physics at a higher level, the advantages seem to outweigh the flaws. This is especially the case considering that those who do go on to learn more will tend to be the students most able to abandon the old model.

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  • $\begingroup$ Honestly, it's possible to get even more from the model than what I listed above. On a much deeper level, the model correctly implies that electrons occupy space that is radially more distant from the center than neutrons and protons; this distance tends to increase with the number of electrons; neutrons and protons are excluded from occupying the same physical space; and chemical reactions are more likely than nuclear ones. On a qualitative level, the model isn't so bad. $\endgroup$
    – AdamRedwine
    Commented Nov 11, 2011 at 15:37
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    $\begingroup$ This means the model is useful, like Plato's « medicinal lie.» It does not indicate that it is valid. I met a philosophy professor who had given up science in high school after she found out that she had been lied to by her earlier teachers using this planetary model...and she didn't agree with Plato either.... $\endgroup$
    – joseph f. johnson
    Commented Jan 16, 2012 at 4:17
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    $\begingroup$ I think I agree with joseph here: the model is useful, but not valid. At the very least, starting out by saying "Yes, the model is as valid as it has ever been" is grossly misleading (even though I see how it could technically be considered a true statement). $\endgroup$
    – David Z
    Commented Jan 16, 2012 at 5:13
  • $\begingroup$ And what model do you then propose is valid? Is it possible to have degrees of validity? If you demand absolute correlation between theory and reality, the standard model of particle physics is surely invalid. The question asked if the model is "still valid." The interpretation of the word "valid" as you are suggesting it would render the question itself meaningless. $\endgroup$
    – AdamRedwine
    Commented Jan 16, 2012 at 13:19
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    $\begingroup$ All models are invalid - some models are useful. That's basically the definition of physics. $\endgroup$
    – Martin Beckett
    Commented Jan 16, 2012 at 16:48
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The model is very invalid, and for all the reasons you mention: chemical bonds are inexplicable in terms of this model, etc. And for other reasons you don't mention but which were understood at the time: a rotating charged particle (such as the electron is supposed to be in this model) generates a radio wave the same way our antennas do, so it loses energy and has to fall into the nucleus within split seconds.

Pedagogy has not come up with a more-valid model that succeeds in all the things which Mr. Redwine correctly points out do get across using the invalid planetary model. But perhaps if we try, something can be done with the de Broglie matter wave model or with Schroedinger's (early, naive, and incorrect) understanding of the electron wave. Such a model is more valid, even when you leave out all mention of probabilities so it's not perhaps completely valid, and I think something could be done with it. See my answer to the linked question.

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  • $\begingroup$ The details of chemical bonds is inexplicable - but a lot of inorganic chemistry is fairly well predicted by simply gaining or losing valence electrons. As models go it's pretty good value for money! $\endgroup$
    – Martin Beckett
    Commented Jan 16, 2012 at 16:49
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Yes, in some cases.

Nearly a century after Danish physicist Niels Bohr offered his planet-like model of the hydrogen atom, a Rice University-led team of physicists has created giant, millimeter-sized atoms that resemble it more closely than any other experimental realization yet achieved.

Using lasers, the researchers excited potassium atoms to extremely high levels. Using a carefully tailored series of short electric pulses, the team was then able to coax the atoms into a precise configuration with one point-like, "localized" electron orbiting far from the nucleus

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    $\begingroup$ Since the electrons only remain localised in the circular orbit for several revolutions, these are not atoms since they suffer from the instability which led Bohr to invent quantum theory in the first place: a rotating charged particle loses energy by electro-magnetic radiation and cannot stay in orbit. So, ahem, it's not really an orbit... $\endgroup$
    – joseph f. johnson
    Commented Jan 16, 2012 at 4:15

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